Controller and drive circuits for electric motors

ABSTRACT

An electric motor system is described. The electric motor system includes a drive circuit configured to supply variable frequency current and a contactor configured to supply line frequency current, wherein the drive circuit includes a three-phase inverter and an H-bridge including two phases of the inverter. The electric motor system also includes an electric motor and a controller. The controller is configured to control the inverter to supply variable frequency current to the electric motor over a first duration and determine to control the drive circuit to transition from supplying variable frequency current to supplying line frequency current. The controller is also configured to determine a polarity of a sensed alternating current (AC) voltage, disable at least two switches of the H-bridge, and control the contactor to close, thereby preventing the contactor and the inverter from energizing the electric motor at the same time once the contactor is closed.

BACKGROUND

The field of the disclosure relates generally to electric motors, andspecifically to a motor controller controlling a drive circuit bysimplifying the process of the motor controller controlling the drivecircuit to transition from using an inverter to supplying line frequencypower.

At least some known electric motors are fixed speed motors that operatemost efficiently at line frequency power. Such motors exhibituncontrolled acceleration during startup. Further, at low loadconditions, such motors operate less efficiently. Alternatively, someinduction motors may be driven with a variable speed motor controller toadapt motor speed to a load level. Such configurations are generallylimited by power factor, electromagnetic interference, and electricallosses.

A drive circuit for certain motors enables efficient operation at bothhigh and low load conditions. For example, a motor operating acompressor in a heating, ventilation and air conditioning (HVAC) systemmay experience high load conditions during peak temperatures and lowload conditions during milder temperatures. The drive circuit operatesthe motor using an inverter under low load conditions, and operates themotor using line frequency power under high load conditions.

Transitioning from using an inverter to supplying line frequency powerpresents significant challenges. For example, compressors may stall orexperience significant loss of speed within one line cycle. Further, acontactor closing to connect a motor to line frequency power may requirebetween one and two line cycles to open/close, thereby causing the motorto stall or experience significant loss of speed during theopening/closing. Further, the contactor and inverter cannot operate atthe same time because of the risk of creating a line to line shortcircuit. Current systems may utilize one or more triodes for alternatingcurrent (TRIACs), or any arrangement of semi-conductor switches offering4 quadrant operation, in order to supply power to the motor duringabove-mentioned contactor closing delay when transitioning from inverterto line frequency power. However, TRIACs, or other suitable arrangementof semi-conductor switches offering 4 quadrant operation, can be costlyand consume valuable surface area in electronics equipment. Accordingly,systems and methods for controlling the drive circuit to safelytransition from using an inverter to supplying line frequency powerwithout the use of additional switches/TRIACs are desired.

BRIEF DESCRIPTION

In one aspect, an electric motor system is described. The electric motorsystem includes a drive circuit configured to supply variable frequencycurrent over a first duration and a contactor configured to supply linefrequency current over a second duration, wherein the drive circuitincludes a three-phase inverter and an H-bridge including two phases ofthe inverter. The electric motor system also includes an electric motorcoupled to the drive circuit, and a controller communicatively coupledto the drive circuit. The controller is configured to control theinverter to supply variable frequency current to the electric motor overthe first duration and determine to control the drive circuit totransition from supplying variable frequency current to supplying linefrequency current. The controller is also configured to determine apolarity of a sensed alternating current (AC) voltage, disable, basedupon the determined polarity of the sensed AC voltage, at least twoswitches of the H-bridge, and control the contactor to close, therebypreventing the contactor and the inverter from energizing the electricmotor at the same time once the contactor is closed.

In another aspect, an electric motor is described. The electric motorincludes a drive circuit configured to supply variable frequency currentover a first duration and a contactor configured to supply linefrequency current over a second duration, wherein the drive circuitincludes a three-phase inverter and an H-bridge including two phases ofthe inverter. The electric motor also includes a controllercommunicatively coupled to the drive circuit. The controller isconfigured to control the inverter to supply variable frequency currentto the electric motor over the first duration and determine to controlthe drive circuit to transition from supplying variable frequencycurrent to supplying line frequency current. The controller is alsoconfigured to determine a polarity of a sensed alternating current (AC)voltage, disable, based upon the determined polarity of the sensed ACvoltage, at least two switches of the H-bridge, and control thecontactor to close, thereby preventing the contactor and the inverterfrom energizing the electric motor at the same time once the contactoris closed.

In yet another aspect, a method of operating an electric motor isdescribed. The electric motor is coupled to a drive circuit that iscommunicatively coupled to a controller, wherein the drive circuit isconfigured to supply variable frequency current over a first durationand a contactor configured to supply line frequency current over asecond duration, wherein the drive circuit includes a three-phaseinverter and an H-bridge including two phases of the inverter. Themethod includes controlling the inverter to supply variable frequencycurrent to the electric motor over the first duration and determining tocontrol the drive circuit to transition from supplying variablefrequency current to supplying line frequency current. The method alsoincludes determining a polarity of a sensed alternating current (AC)voltage, disabling, based upon the determined polarity of the sensed ACvoltage, at least two switches of the H-bridge, and controlling thecontactor to close, thereby preventing the contactor and the inverterfrom energizing the electric motor at the same time once the contactoris closed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary drive circuit for anelectric motor;

FIG. 2 is a schematic diagram of another exemplary drive circuit for anelectric motor in accordance with the present disclosure; and

FIG. 3 is a flow diagram of an exemplary method of operating an electricmotor in accordance with the present disclosure.

FIGS. 4-7 are diagrams of exemplary results produced by the exemplarydrive circuit for an electric motor in accordance with the presentdisclosure.

DETAILED DESCRIPTION

In operating an electric motor (e.g., and a mechanical compressor drivenby the electric motor), a drive circuit for the electric motor drivesthe electric motor with an inverter under low load conditions and withline frequency power under high load conditions. Generally, the inverterand line frequency power cannot both be connected to the electric motorat the same time, because of the potential for a line-to-line shortcircuit. To transition from inverter to line, or line to inverter, oneis disconnected before connecting the other.

When transitioning from the inverter to line frequency power, acontactor closing to connect a motor to line frequency power may requirebetween one and two line cycles to open/close, thereby causing the motorto stall or experience significant loss of speed during theopening/closing. Further, the contactor and inverter cannot operate atthe same time because of the risk of creating a line to line shortcircuit.

Accordingly, the drive circuit described herein is in communication withat least one controller (e.g., a motor controller, a system controller,etc.) that controls, during the transition from the inverter to linefrequency power, power supply to the motor during the above-mentionedcontactor closing delay without requiring the use of costly andspace-taking additional solid state switches such as TRIACs.

In the example embodiment, the at least one controller is configured,when controlling the transition from supplying variable frequencycurrent to supplying line frequency current, to disable a third leg ofthe inverter (e.g., wherein the first two legs of the inverter form anH-bridge), commutate a start winding of the electric motor, synchronizemotor speed to a desired level so as to minimize motor slip during thetransition, and/or minimize inverter current prior to the transition,and synchronize the phase of motor voltage to a desired level in orderto minimize out of phase current while transitioning from inverter toline operation. Further, to control power supply during theabove-mentioned contactor delay, the controller is configured todetermine a polarity of a sensed alternating current (AC) voltage,disable, based upon the determined polarity of the sensed AC voltage, atleast two switches of the H-bridge, and control the contactor to close,thereby preventing the contactor and the inverter from energizing themotor at the same time. Controlling the contactor to close may includecontrolling the contactor to begin closing and controlling pulse widthmodulation (PWM) at the electric motor (e.g., controlling duty cycle,frequency, amplitude, etc. of power delivered to the electric motor)until the contactor is closed (e.g., after 1.5 line cycles arecomplete).

FIG. 1 is a schematic diagram of an exemplary drive circuit 100 for aPSC motor 102 including solid state switches 130 (e.g., TRIACs) similarto some known systems. PSC motor 102 includes a start winding 104 and amain winding 106. During normal line frequency operation, line frequencycurrent, such as 50 Hertz or 60 Hertz, for example, is supplied on afirst line, or L1, 108 to start winding 104 through a capacitor 110, andto main winding 106. A second line, or L2, 112 provides a return, orneutral, for the line frequency current. Drive circuit 100 includes acontactor 114 for connecting and disconnecting L1 and L2 to PSC motor102. Contactor 114 may include a two pole mechanical contactor that iscommutated by energizing a coil (not shown). In certain embodiments,capacitor 110 may be coupled to L1 on either side of contactor 114.

Drive circuit 100 includes an inverter 116 that is enabled to drive PSCmotor 102 with variable frequency power under low load, or at least lessthan full load, conditions. In some embodiments, inverter 116 issupplied line frequency power on L1 and L2, and is controlled overcontrol lines 118 and 120, or Y1 and Y2. In some embodiments, inverter116 may be controlled by any other suitable means, including, forexample, digital control signals (e.g., serial communication or Modbuscommunication) and analog control signals (e.g., transmitted from motorcontroller 132 or system controller 140). Inverter 116 enables variablespeed operation of PSC motor 102 by regulating phase and frequency ofalternating current (AC) voltages on output terminals W, U, and V.Terminal W is coupled to a node 122, terminal U is coupled to a node124, and terminal V is coupled to a node 126. Drive circuit 100 includesa bypass switch 128 that enables bypass of capacitor 110 duringoperation through inverter 116. When driven by inverter 116, startwinding 104 of PSC motor 102 is coupled across nodes 122 and 126, i.e.,terminals W and V of inverter 116, and main winding 106 is coupledacross nodes 124 and 126, i.e., terminals U and V of inverter 116.

When operating PSC motor 102 using inverter 116, contactor 114 is openand inverter 116 is enabled via control lines 118 and 120, or othersuitable control means. To transition to line frequency power, inverter116 is disabled and contactor 114 is closed to couple L1 and L2 directlyto PSC motor 102. Contactor 114 may require one to two line cycles toclose.

In some known systems, drive circuit 100 includes solid state switches130 coupled in parallel with the two poles of contactor 114 on L1 andL2. During the transition from inverter 116 to line frequency power, andafter inverter 116 is disabled, solid state switches 130 are closed tocouple L1 and L2 directly to PSC motor 102. Solid state switches 130remain closed and conduct line frequency current until contactor 114 isclosed. Once contactor 114 is closed, solid state switches 130 areopened to redirect the line frequency current through contactor 114. Insome embodiments, contactor 114 and solid state switches 130 areconnected to L1 and L2 through an input impedance (not shown) ofinverter 116 (e.g., after EMI filter and inrush current limiter).However, solid state switches, such as TRIACs, can be costly and consumevaluable surface area in electronics equipment. Accordingly, systems andmethods for controlling the drive circuit to safely transition fromusing an inverter to supplying line frequency power without the use ofthese solid state switches are desired (e.g., as described with respectto FIG. 2).

Motor controller 132 is communicatively coupled to motor 102 to operatemotor 102. Further, motor controller 132 may be coupled to one or moreadditional components of drive circuit 100, including inverter 116,contactor 114, solid state switches 130, and control lines 118 and 120(Y1 and Y2). More specifically, motor controller 132 transmits controlsignals to operate motor 102. In the example embodiment, by adjustingthe control signals, motor controller 132 is configured to controlinverter 116 to supply variable frequency current to motor 102 asdescribed above. Further, motor controller 132 is configured to adjustthe control signals to control the transition from inverter 116supplying variable frequency power, to supplying line frequency power asdescribed above (e.g., from L1 and L2). For example, in someembodiments, motor controller 132 and inverter 116 are embodied in thesame drive (e.g., such that motor controller 132 receives signals fromcontrol lines 118 and 120, and then commands inverter 116 based upon thereceived signals). In some embodiments, motor controller 132 may becommunicatively coupled to another controller (e.g., system controller140) associated with motor 102. In such embodiments, motor controller132 may be configured to allow system controller 140 to operate motor102. In the exemplary embodiment, motor controller 132 is separate frommotor 102. In one example, motor controller 132 and inverter 116 may beintegrated with motor 102. In another example, motor controller 132and/or system controller 140 is an external controller, such as athermostat system controller. In some embodiments, motor controller 132and system controller 140 may be integrated in the same controller(e.g., any description of motor controller 132 may be integrated insystem controller 140, or any other controller, and vice versa).

In an example embodiment, motor controller 132 includes processor 134,memory 136 communicatively coupled to processor 134, and communicationsinterface 138. Motor controller 132 is also communicatively coupledsystem controller 140. Processor 134 is configured to executeinstructions stored within memory 136 to cause motor controller 132 tofunction as described herein. Moreover, memory 136 is configured tostore data to facilitate controlling motor 102. In some embodiments,motor controller 132 may include a plurality of processors 134 and/ormemories 136. In other embodiments, memory 136 may be integrated withprocessor 134. In one example, memory 136 includes a plurality of datastorage devices to store instructions and data as described herein.Communications interface 138 may include one or more wired or wirelesshardware interface such as, for example, universal serial bus (USB),RS232, RS485, or other serial bus, CAN bus, Ethernet, near fieldcommunication (NFC), WiFi, Bluetooth, or any other suitable digital oranalog interface for establishing one or more communication channels.The established communication channels may include, for example,channels between motor controller 132 and system controller 140.Communications interface 138 further includes a software or firmwareinterface for receiving one or more motor control parameters and writingthem, for example, to memory 136. In some embodiments, communicationinterface 138 includes, for example, a software application programminginterface (API) or command set for controlling, as an example, acontactor to close when transitioning from using an inverter tosupplying line frequency power.

In the exemplary embodiment, system controller 140 includes processor142, memory 144 communicatively coupled to processor 142, andcommunications interface 146. System controller 140 is alsocommunicatively coupled motor controller 132. Processor 142 isconfigured to execute instructions stored within memory 144 to causesystem controller 140 to function as described herein. In someembodiments, system controller 140 may include a plurality of processors134 and/or memories 136. In other embodiments, memory 144 may beintegrated with processor 142. In one example, memory 144 includes aplurality of data storage devices to store instructions and data asdescribed herein. Communications interface 146 may include one or morewired or wireless hardware interface such as, for example, universalserial bus (USB), RS232 or other serial bus, CAN bus, Ethernet, nearfield communication (NFC), WiFi, Bluetooth, or any other suitabledigital or analog interface for establishing one or more communicationchannels. The established communication channels may include, forexample, channels between system controller 140 and motor controller132. Communications interface 146 further includes a software orfirmware interface for receiving one or more motor control parametersand writing them, for example, to memory 144. As explained above, insome embodiments, system controller 140 may be configured to perform anyof the functions described herein with respect to motor controller 132or any other controller.

FIG. 2 is a schematic diagram of drive circuit 200 for an electric motor202, such as motor 102, without requiring the use of solid stateswitches 130 (e.g., TRIACs) while maintaining the reliable functionalityof constantly providing current to motor 202 (e.g., specifically duringtransitioning from using an inverter to supplying line frequency power).During normal line frequency operation, line frequency current, such as50 Hertz or 60 Hertz, for example, is supplied on a first line, or L1,204, through a run capacitor 234 (e.g., capacitor 110), to a mainwinding 208, and to a start winding 210. The terms line frequencycurrent, voltage, and/or power are used interchangeably herein to referto direct electrical communication with AC source 232. A second line, orL2, 206 provides a return, or neutral, for the line frequency current.Drive circuit 200 includes a contactor 212 for connecting anddisconnecting L1 and L2 to the PSC motor. Contactor 212 is a two polemechanical contactor that is commutated by energizing a coil (notshown). In certain embodiments, a run capacitor may be coupled to L1 oneither side of contactor 212. In some embodiments, a relay 236 may becoupled between the run capacitor and start winding 210.

Drive circuit 200 includes an inverter 214 (e.g., inverter 116) that isenabled to drive electric motor 202 with variable frequency power underlow load, or at least less than full load, conditions. Inverter 214 issupplied line frequency power on L1 and L2. Inverter 214 enablesvariable speed operation of electric motor 202 by regulating amplitude,phase, and frequency of alternating current (AC) voltages on outputterminals thereof, which are coupled to main winding 208 and startwinding 210. When operating electric motor 202 using inverter 214,contactor 212 is open and inverter 214 is enabled via any suitablecontrol means. To transition to line frequency power, at least a portionof inverter 214 is disabled (e.g., inverter 214 is operated in H-bridgemode, as described herein), contactor 212 is closed, and relay 236 maybe commutated to couple L1 and L2 directly to electric motor 202.

Although electric motor 202 is illustrated as a PSC motor, it isrecognized that other known motors (such as electronically commutatedmotors (ECMs)) also have integrated windings (e.g., between windings ofa three-phase ECM). Electric motor 202 may be an induction motor, suchas a PSC motor, or a permanent magnet motor, such as an ECM. Moreover,electric motor 202 may drive a compressor, or may drive any otherfluid-moving apparatus, such as a fan, blower, impeller, pump, and thelike.

Drive circuit 200 includes a rectifier 216, inverter 214 downstream fromrectifier 216 and contactor 212. Contactor 212 may be embodied asmechanical/electromechanical contactors, electronic switches, and/or orsolid-state switches. Under the first mode of operation, contactor 212is open, and drive circuit 200 is configured to drive motor 202 usinginverter 214. Inverter 214 enables variable speed operation of motor 202by regulating current provided to main winding 208 and start winding210, by controlling amplitude, phase, and frequency of current andvoltage on output terminals thereof, which are coupled to main winding208 and start winding 210.

In the example embodiment, inverter 214 includes a capacitor 218 and aplurality of switches arranged in three parallel sets of switches 220,222, 224, also referred to as phases 226, 228, and 230, respectively, ofinverter 214 (e.g., first set of switches 220 may be referred to as afirst phase 226 of inverter, second set of switches 222 may be referredto as a second phase 228 of inverter 214, and third set of switches 224may be referred to as a third phase 230 of inverter 214). Switches 220,222 (e.g., and phases 226, 228) may be referred to as an H-bridge ofinverter 214. When operating at variable speed frequency, start winding210 is coupled to second phase 228 and is coupled to third phase 230through bypass switch/relay 236.

Rectifier 216 rectifies power from AC source 232, capacitor 218functions as a storage element for the rectified power from rectifier216, and sets of switches 220, 222, 224 (phases 226, 228, and 230) toregulate current provided to windings 208, 210 in some modes ofoperation.

Contactor 212 may be controlled (e.g., closed, opened, commutated) byany suitable control means, such as, for example, a microcontroller, afield programmable gate array (FPGA), a digital signal processing (DSP)device, a remote system controller, a local system controller, and thelike (e.g., motor controller 132 and system controller 140). Contactor212 may be controlled to enable switching between supplying variablefrequency power and line frequency power (e.g., driving second winding210 using inverter 214 or directly with line frequency voltage andcurrent from AC source 232).

In addition, drive circuit 200 has a simplified wiring scheme, comparedto drive circuit 100 and other known drive circuits. In particular,drive circuit 200 includes a reduced number of wiring connections (e.g.,because of the elimination of solid state switches 130). Further, drivecircuit 200 is less costly and requires less surface area than drivecircuit 100 (e.g., because of the elimination of solid state switches130).

Accordingly, drive circuit 200 described herein is in communication withat least one controller (e.g., motor controller 132, system controller140, etc.) and controls, during the transition from the inverter to linefrequency power, power supply to motor 202 during the above-mentionedcontactor 212 closing delay without requiring the use of costly andspace-taking additional solid state switches such as TRIACs (e.g., solidstate switches 130) while maintaining the reliable functionality ofconstantly providing current to motor 202 (e.g., specifically duringtransitioning from using inverter 214 to supplying line frequencypower).

In the example embodiment, the at least one controller is configured tooperate inverter 214 in an H-bridge mode, when controlling thetransition from supplying variable frequency current to supplying linefrequency current, to disable third phase 230 of inverter 214, commutatestart winding 210 of electric motor 202 through bypass switch/relay 236,synchronize motor speed to a desired level so as to minimize motor slipduring transition, and/or minimize inverter current prior to transition,and synchronize the phase of motor voltage to a desired level in orderto minimize out of phase current while transitioning from inverter toline operation. Further, to control power supply during theabove-mentioned contactor delay, the controller is configured todetermine a polarity of a sensed alternating current (AC) voltage,disable, based upon the determined polarity of the sensed AC voltage, atleast two switches of the H-bridge (e.g., the lower switch of switches220 and the upper switch of switches 222), and control contactor 212 toclose, thereby preventing contactor 212 and inverter 214 from energizingmotor 202 at the same time once the contactor reaches its fully-closedstate. In some embodiments, based upon the determined polarity of thesensed AC voltage, at least two different switches of the H-bridge maybe disabled (e.g., the upper switch of switches 220 and the lower switchof switches 222). Controlling contactor 212 to close may includecontrolling contactor 212 to begin closing and controlling pulse widthmodulation (PWM) at electric motor 202 until contactor 212 is closed(e.g., after 1.5 line cycles are complete).

FIG. 3 is a flow diagram of an exemplary method 300 of operating anelectric motor (e.g., electric motor 202, shown in FIG. 2), such as aninduction motor or a permanent magnet motor. Method 300 includescontrolling 310 an inverter (e.g., inverter 214) to supply variablefrequency current to the electric motor over the first duration anddetermining 320 to control a drive circuit (e.g., drive circuit 200) totransition from supplying variable frequency current to supplying linefrequency current. Method 300 also includes determining 330 a polarityof a sensed alternating current (AC) voltage, disabling 340, based uponthe determined polarity of the sensed AC voltage, at least two switchesof an H-bridge (e.g., H-bridge of inverter 214). Method 300 furtherincludes controlling 350 a contactor (e.g., contactor 212) to close,thereby preventing the contactor and the inverter from energizing theelectric motor at the same time.

In some embodiments, method 300 includes controlling the contactor tobegin closing and controlling pulse width modulation (PWM) at theelectric motor until the contactor is closed. In some embodiments,method 300 includes controlling PWM at the electric motor until thecontactor is closed by controlling PWM at the electric motor for atleast 1.5 line cycles. In some embodiments, method 300 includesdetermining the contactor is closed and controlling PWM at the electricmotor until the controller determines that the contactor is closed. Insome embodiments, method 300 includes disabling a third leg of theinverter and commutating a start winding of the electric motor. In someembodiments, method 300 includes controlling motor speed of the electricmotor to be synchronized to a desired speed and controlling a phase ofmotor voltage to be synchronized to a desired phase.

FIGS. 4-7 are diagrams (400, 500, 600, and 700) of exemplary resultsproduced by the exemplary drive circuit for an electric motor inaccordance with the present disclosure. Diagram 400 illustratesexemplary AC voltage and PWM signals, while diagram 500 illustrates thatcurrent at a motor (e.g., motor 202) remains nearly sinusoidal for 1.5line cycles while utilizing only two switches (e.g., the lower switch ofswitches 220 and the upper switch of switches 222) during the transitionfrom inverter power to AC (line) power. Accordingly, the resultingcompressor current shown in diagram 600 remains nearly sinusoidal whenthe contactor (e.g., contactor 212) is unenergized (open) and when thecontactor is energized (closed) as shown in diagram 700. Thus, thesystems and methods described herein result in a quasi-seamlesstransition from supplying inverter (e.g., inverter 214) power tosupplying AC power—without dedicated line synchronization circuitry(e.g., additional switches/TRIAC s).

Some embodiments involve the use of one or more electronic or computingdevices (e.g., for controlling operation of a drive circuit and/orindividual components thereof). Such devices typically include aprocessor, processing device, or controller, such as a general purposecentral processing unit (CPU), a graphics processing unit (GPU), amicrocontroller, a reduced instruction set computer (RISC) processor, anapplication specific integrated circuit (ASIC), a programmable logiccircuit (PLC), a field programmable gate array (FPGA), a digital signalprocessing (DSP) device, and/or any other circuit or processing devicecapable of executing the functions described herein. The methodsdescribed herein may be encoded as executable instructions embodied in acomputer readable medium, including, without limitation, a storagedevice and/or a memory device. Such instructions, when executed by aprocessing device, cause the processing device to perform at least aportion of the methods described herein. The above examples areexemplary only, and thus are not intended to limit in any way thedefinition and/or meaning of the terms processor, processing device, andcontroller.

In the embodiments described herein, memory may include, but is notlimited to, a computer-readable medium, such as a random access memory(RAM), and a computer-readable non-volatile medium, such as flashmemory. Alternatively, a floppy disk, a compact disc—read only memory(CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc(DVD) may also be used. Also, in the embodiments described herein,additional input channels may be, but are not limited to, computerperipherals associated with an operator interface such as a mouse and akeyboard. Alternatively, other computer peripherals may also be usedthat may include, for example, but not be limited to, a scanner.Furthermore, in the exemplary embodiment, additional output channels mayinclude, but not be limited to, an operator interface monitor.

As used herein, an element or step recited in the singular and precededwith the word “a” or “an” should be understood as not excluding pluralelements or steps, unless such exclusion is explicitly recited.Furthermore, references to “example implementation” or “oneimplementation” of the present disclosure are not intended to beinterpreted as excluding the existence of additional implementationsthat also incorporate the recited features.

As used herein, the terms “software” and “firmware” are interchangeable,and include any computer program stored in memory for execution by aprocessor, including RAM memory, ROM memory, EPROM memory, EEPROMmemory, and non-volatile RAM (NVRAM) memory. The above memory types areexamples only, and are thus not limiting as to the types of memoryusable for storage of a computer program.

The systems and methods described herein are not limited to the specificembodiments described herein, but rather, components of the systemsand/or steps of the methods may be utilized independently and separatelyfrom other components and/or steps described herein.

This written description uses examples to provide details on thedisclosure, including the best mode, and also to enable any personskilled in the art to practice the disclosure, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the disclosure is defined by the claims, and mayinclude other examples that occur to those skilled in the art. Suchother examples are intended to be within the scope of the claims if theyhave structural elements that do not differ from the literal language ofthe claims, or if they include equivalent structural elements withinsubstantial differences from the literal language of the claims.

What is claimed is:
 1. An electric motor system comprising: a drive circuit configured to supply variable frequency current over a first duration and a contactor configured to supply line frequency current over a second duration, wherein the drive circuit includes a three-phase inverter and an H-bridge comprising two of the phases of the inverter; an electric motor coupled to the drive circuit; and a controller communicatively coupled to the drive circuit, wherein the controller configured to: control the inverter to supply variable frequency current to the electric motor over the first duration; determine to control the drive circuit to transition from supplying variable frequency current to supplying line frequency current; determine a polarity of a sensed alternating current (AC) voltage; disable, based upon the determined polarity of the sensed AC voltage, at least two switches of the H-bridge; and control the contactor to close, thereby preventing the contactor and the inverter from energizing the electric motor at the same time once the contactor is closed.
 2. The electric motor system of claim 1, wherein the controller is further configured to, in controlling the contactor to close: control the contactor to begin closing; and control pulse width modulation (PWM) at the electric motor until the contactor is closed.
 3. The electric motor system of claim 2, wherein the controller is further configured to control PWM at the electric motor until the contactor is closed by controlling PWM at the electric motor for at least 1.5 line cycles.
 4. The electric motor system of claim 2, wherein the controller is further configured to: determine the contactor is closed; and control PWM at the electric motor until the controller determines that the contactor is closed.
 5. The electric motor system of claim 1, wherein the controller is further configured to, before determining a polarity of a sensed alternating current (AC) voltage: disable a third phase of the inverter; and commutate a start winding of the electric motor.
 6. The electric motor system of claim 1, wherein the controller is further configured to: control motor speed of the electric motor to be synchronized to a desired speed; and control a phase of motor voltage to be synchronized to a desired phase.
 7. The electric motor system of claim 1, wherein the controller is further determine to control the drive circuit to transition from supplying variable frequency current to supplying line frequency current based upon at least one input signal received at the controller.
 8. An electric motor comprising: a drive circuit configured to supply variable frequency current over a first duration and a contactor configured to supply line frequency current over a second duration, wherein the drive circuit includes a three-phase inverter and an H-bridge comprising two phases of the inverter; and a controller communicatively coupled to the drive circuit, wherein the controller configured to: control the inverter to supply variable frequency current to the electric motor over the first duration; determine to control the drive circuit to transition from supplying variable frequency current to supplying line frequency current; determine a polarity of a sensed alternating current (AC) voltage; disable, based upon the determined polarity of the sensed AC voltage, at least two switches of the H-bridge; and control the contactor to close, thereby preventing the contactor and the inverter from energizing the electric motor at the same time once the contactor is closed.
 9. The electric motor of claim 8, wherein the controller is further configured to, in controlling the contactor to close: control the contactor to begin closing; and control pulse width modulation (PWM) at the electric motor until the contactor is closed.
 10. The electric motor of claim 9, wherein the controller is further configured to control PWM at the electric motor until the contactor is closed by controlling PWM at the electric motor for at least 1.5 line cycles.
 11. The electric motor of claim 9, wherein the controller is further configured to: determine the contactor is closed; and control PWM at the electric motor until the controller determines that the contactor is closed.
 12. The electric motor of claim 8, wherein the controller is further configured to, before determining a polarity of a sensed alternating current (AC) voltage: disable a third phase of the inverter; and commutate a start winding of the electric motor.
 13. The electric motor of claim 8, wherein the controller is further configured to: control motor speed of the electric motor to be synchronized to a desired speed; and control a phase of motor voltage to be synchronized to a desired phase.
 14. The electric motor of claim 8, wherein the controller is further determine to control the drive circuit to transition from supplying variable frequency current to supplying line frequency current based upon at least one input signal received at the controller.
 15. A method of operating an electric motor, wherein the electric motor is coupled to a drive circuit that is communicatively coupled to a controller, wherein the drive circuit is configured to supply variable frequency current over a first duration and a contactor configured to supply line frequency current over a second duration, wherein the drive circuit includes a three-phase inverter and an H-bridge comprising two phases of the inverter, said method comprising: controlling the inverter to supply variable frequency current to the electric motor over the first duration; determining to control the drive circuit to transition from supplying variable frequency current to supplying line frequency current; determining a polarity of a sensed alternating current (AC) voltage; disabling, based upon the determined polarity of the sensed AC voltage, at least two switches of the H-bridge; and controlling the contactor to close, thereby preventing the contactor and the inverter from energizing the electric motor at the same time once the contactor is closed.
 16. The method of claim 15, further comprising, in controlling the contactor to close: controlling the contactor to begin closing; and controlling pulse width modulation (PWM) at the electric motor until the contactor is closed.
 17. The method of claim 16, further comprising controlling PWM at the electric motor until the contactor is closed by controlling PWM at the electric motor for at least 1.5 line cycles.
 18. The method of claim 16, further comprising: determining the contactor is closed; and controlling PWM at the electric motor until the controller determines that the contactor is closed.
 19. The method of claim 15, further comprising, before determining a polarity of a sensed alternating current (AC) voltage: disabling a third phase of the inverter; and commutating a start winding of the electric motor.
 20. The method of claim 15, further comprising: controlling motor speed of the electric motor to be synchronized to a desired speed; and controlling a phase of motor voltage to be synchronized to a desired phase. 